1. Field of the Invention
The present invention generally relates to a CMOS image sensor, and more particularly to a pixel readout circuit of the CMOS image sensor with substantially reduced area, and to a pixel readout circuit with a feedback or switch capacitor.
2. Description of the Prior Art
A complementary metal-oxide-semiconductor (CMOS) image sensor is an electronic device that acquires image information in, for example, a camera by transforming light intensity into charge, which is then converted to a voltage and finally read out. FIG. 1A shows a passive pixel sensor (PPS), which can be considered an early-genre CMOS image sensor. In the figure, only two pixels of a pixel array are shown for illustration purposes. Each pixel contains a photodiode D and an access transistor (or switch) Macc. A word line (e.g., WL1) is connected to the pixels in the same row, and a bit line (e.g., BL) is connected to the pixels in the same column. At the end of each bit line BL is an amplifier 10.
FIG. 1B shows an exemplary pixel circuit of an active pixel sensor (APS). Each pixel contains a photodiode D and three transistors—Mrst, Msf, and Msel, and thus this type of sensor is also known as three-transistor (3T) pixel circuit of the CMOS image sensor. The transistor Mrst, when it is turned on by a reset signal RST, is used to reset the photodiode D to a reset reference voltage, such as that of power supply VDD. The transistor Msf acts as a source follower which buffers or amplifies the integrated light signal of the photodiode D. The transistor Msel, when turned on by a word line signal WL, allows for read out of the pixel signal. The 3T pixel circuit alleviates a “stripe defect” of the passive pixel sensor due to random distribution of noise by the transistors Msf and Mrst. Nevertheless, the 3T pixel circuit suffers from high leakage current in its photodiode D. Moreover, even to the extent the stripe defect can be improved, the transistor Mrst introduces KT/Cp noise, which is relatively large since Cp, the parasitic capacitance, is very small.
FIG. 1C shows an exemplary pixel circuit of another active pixel sensor (APS). Each pixel contains a photodiode D and four transistors (Mtx, Mrst, Msf, and Msel), and accordingly this type of sensor is also known as a four-transistor (4T) pixel circuit of the CMOS image sensor. The 4T pixel circuit is arranged and functions similar to the 3T pixel circuit, except that an additional transistor Mtx, when turned on by a transfer signal TX, is used to controllably transfer the integrated light signal of the photodiode D. The 4T pixel circuit is capable of advantageously facilitating correlated double sampling (CDS) in the CMOS image sensor to eliminate the different characteristics across the pixels due to process variation. Also, the photodiode is designed to fully transfer integrated charges to the outside providing the potential and capacity of the floating diffusion FD are sufficient. Thus, by doing a correlated double sampling readout, KT/Cp noise can be totally removed, resulting in a very low temporal noise level. Moreover, the lack of contact inside the photodiode also contributes to a much lower dark current.
The pixel circuit of FIG. 1C, however, occupies substantial chip area, and is thus not adaptable to modern and high density CMOS image sensors. For the foregoing reason, a need has arisen to propose a pixel readout circuit of the CMOS image sensor that can effectively reduce an overall size of the entire pixel array of the CMOS image sensor.